The Multidimensional Cube Recurrence

TL;DR

The paper introduces the multidimensional cube recurrence, a generalization of known recurrences, demonstrating its independence from flip order, Laurent phenomenon, and applications to Grassmannians and tropical inequalities.

Contribution

It generalizes previous recurrences, proves their independence from flip order, and connects to Grassmannians and tropical geometry.

Findings

Recurrence values are independent of flip order.

Variables are Laurent polynomials of initial variables.

Special case describes equations for isotropic Grassmannians.

Abstract

We introduce a recurrence which we term the multidimensional cube recurrence, generalizing the octahedron recurrence studied by Propp, Fomin and Zelevinsky, Speyer, and Fock and Goncharov and the three-dimensional cube recurrence studied by Fomin and Zelevinsky, and Carroll and Speyer. The states of this recurrence are indexed by tilings of a polygon with rhombi, and the variables in the recurrence are indexed by vertices of these tilings. We travel from one state of the recurrence to another by performing elementary flips. We show that the values of the recurrence are independent of the order in which we perform the flips; this proof involves nontrivial combinatorial results about rhombus tilings which may be of independent interest. We then show that the multidimensional cube recurrence exhibits the Laurent phenomenon -- any variable is given by a Laurent polynomial in the other variables. We recognize a special case of the multidimensional cube recurrence as giving explicit equations for the isotropic Grassmannians IG(n-1,2n). Finally, we describe a tropical version of the multidimensional cube recurrence and show that, like the tropical octahedron recurrence, it propagates certain linear inequalities.